The invention relates to a method for operating a direct oxidation fuel cell in which the fuel cell is supplied generally with methanol via a transport device for the fuel. The invention likewise relates to a corresponding arrangement comprising a direct oxidation fuel cell, a fuel reservoir and at least one device for transporting the fuel through the fuel cell.
The energy requirement of electrical small consumers (PDA, mobile phones etc.) is constantly increasing. However whilst microprocessors have become approx. thirty times faster in the past ten years, the energy density of batteries has merely doubled. The fuel cell is regarded in this context as a possible replacement or as a supplement for conventional batteries and accumulators. In particular direct oxidation fuel cells (e.g. direct methanol fuel cells) are seen as particularly promising mobile energy sources since the liquid fuel can be handled comparatively easily and generally has an energy density higher by a multiple than batteries or accumulators.
One of the essential challenges for operation of a direct oxidation fuel cell resides in the production of carbon dioxide bubbles during the oxidation of the carbon-containing, liquid fuel on the anode side. There may be mentioned as example the oxidation of methanol with the help of water: CH3OH+H2O→CO2+6H*+6e. These bubbles can become fixed within the gas diffusion layer or in the fluid distribution structure and thus reduce the active cell surface and the performance of the fuel cell.
In a standard mode of operation, a pump which provides the direct oxidation fuel cell with liquid fuel is operated continuously. The gas bubbles are dissolved in liquid with the generally superstoichiometric volume flow or are rinsed out of the fuel cell with the liquid and are separated in a subsequent step from the liquid fuel. This requires continuous operation of the pump which is accompanied with fairly large power requirements of the pump and consequently reduces the efficiency of the system of the entire fuel cell system.